1. Field of the Invention
The present invention relates to an ultrasound measurement system which comprises an ultrasound transducer and permits accurate time-of-flight measurements of ultrasound pulses in connecting elements which are under mechanical stress, for example screws, bolts, rivets or the like.
2. Description of the Background Art
DE 42 25 035 A1 and DE 42 32 254 A1 disclose ultrasound testing methods. DE 10 2004 038 638 relates to a connecting component, in the head or foot region of which there is an ultrasound sensor whose structure is formed as a layer structure.
DE 10 2009 060 441 A1, which corresponds to US2011/0146412, and which is incorporated herein by reference, relates to a sensor element. A connecting component comprises an integrated ultrasound sensor for analyzing the mechanical stress distribution, in particular for determining the prestress force of the connecting component, the ultrasound sensor having at least a two-layered structure comprising an electrode layer and at least one layer of a material having piezoelectric properties. The electrode layer and the layer of a material having piezoelectric properties are vapor deposited, in particular sputtered, on at least one freely accessible end of the connecting part, structures being formed in the electrode layer by generating laser-ablated regions.
The most important field of ultrasound measurement methods is to be seen in the examination of mechanical stress statuses, in particular measurement of the prestress force in connecting elements of different designs. A further very wide field is the “health monitoring” of connecting elements and components which are exposed to dynamic loads. The basis of these measurements includes the excitation, propagation and detection of mechanical ultrasound waves in the materials to be examined, which pass through the regions of the components which are under load or stress while changing their properties in a characterizable way. Especially in the case of force measurement, the difference in the times of flight of an ultrasound pulse between a connecting element which is under mechanical stress and an unloaded reference status is determined. By the mechanical stress, both a lengthening of the connecting element is induced and the speed of sound in the material is reduced. An increase in the sound time of flight can therefore be measured, which can in turn be converted into forces by means of empirical calibration parameters.
The aforementioned methods for equipping connecting elements with adhesively bonded or sputtered transducers places a converter, which converts electrical signals and waveforms into mechanical ultrasound pulses and vice versa, directly on the connecting element. For exciting the transducer and registering the echo, a combination of digital and analog electronics is necessary, which are connected to the transducer by means of cables. For data analysis, data storage and representation of the measurement results, a computer integrated into the measurement unit is used. The pulse transmission and echo reception electronics are applied together with the evaluation computer in a common housing or slot-in rack. In order to avoid interference of the measurement signals and to limit the signal attenuation, the integrated measurement unit itself must be placed in the vicinity of the measurement position.